In an immersion coating process, the hollow object being coated is simply immersed in a coating bath which contains a pool of coating composition. As the hollow object is submersed in the coating composition, the coating flows into the object and adheres to its inner and outer surfaces. With an electro-immersion process, a potential is created between the object being coated and the coating composition, thereby causing the coating to preferentially adhere to inner and outer surfaces of the object being coated. Electro-coating is generally advantageous to simple immersion coating as it creates a physical attraction between the coating composition and the object being coated to ensure adequate coating.
Hollow objects such as open cans are typically electro-immersion coated and several patents discuss this process, including Jorgens U.S. Pat. No. 5,474,610. One problem with coating cans in an immersion process is that they are opened at only one end during the coating process. Thus, removing trapped air from within the cans as they are immersed in the coating is problematic, as is removing the coating from the can as it is withdrawn from the coating bath.
To address the problems associated with coating of hollow cans, the Jorgens reference discloses a method wherein the cans are sequentially transported through a coating bath by a double-sided conveyor system having upper and lower runs in contact with the open tops and closed bottoms of the cans. As the cans are immersed in the coating bath by the conveyor apparatus, a jet spray of coating composition is used to force coating material into the open tops of the cans, thereby displacing trapped air within the cans. The cans continue through the coating bath and pass underneath a second series of coating jet nozzles which further force coating composition into the can to ensure adequate coating. Finally, as the cans are withdrawn from the coating bath, air is injected into the can to ensure that all of the coating composition is removed.
An alternate method of addressing problems associated with coating hollow cans is to simply introduce the cans into the coating bath with the opening of the cans facing upwardly relative to a surface of the coating bath. As the cans are brought into the coating bath, coating liquid will flow into open tops of the cans and displace trapped air within the cans. The cans are then withdrawn from the coating bath with the opening of the cans facing downwardly relative to the coating bath surface to allow excess coating composition to pour out of the cans.
The conveyor apparatus disclosed in Jorgens does not provide the advantageous orientation of the open tops of the cans relative to the coating bath as it relies upon upper and lower conveyor belts to hold the cans in position to prevent them from floating away in the coating bath. With such a conveyor system, the upper and lower belts must move at the same speed or the cans will become tilted or even dislodged. For this reason, it is impossible for the apparatus disclosed in the Jorgens reference to transport cans and rotate them to the degree necessary to ensure that the openings are in the proper orientation as they enter and leave the coating bath to allow the coating to flow into and pour out of the can. This would require a change in the arc of the conveyor belts as they move, which in turn would cause them to move at different speeds.
The Jorgens reference refers to European Patent Application 0118756 which does disclose an apparatus which accomplishes upward orientation of the open tops of the cans as they enter the coating bath and downward orientation of the open tops as they are withdrawn from the bath. However, as the Jorgens reference indicates, the European patent application discloses a relatively complex circular conveyor system which grasps and manipulates the individual cans as they pass through the immersion bath. While such a conveyor apparatus does provide the advantageous orientation of the cans relative to the surface of the coating bath, the conveyor apparatus disclosed in the European patent application is generally not practical for rapid coating of a large number of cans.